You said it yourself: >Why keep a slacker around when you have limited slots for workers? Why not measure and retain good workers? This overall attitude doesn

You said it yourself: >Why keep a slacker around when you have limited slots for workers? Why not measure and retain good workers? This overall attitude doesn't distinguish between "slacker because they don't feel like working" and "slacker because they're unable to work to the degree an able-bodied person can". Your premise reduces someone to their profit-generating ability alone.